Metal processing and forming is an important industrial process. It is often desirable to form or cast a metal product from a metal that is in a molten state that is not entirely liquid. At the same time, it is desirable to control selected properties of the material, such as viscosity. One known process of controlling the material properties consists of shearing a molten alloy with a stirring apparatus, while at the same time cooling the metal through the semi-solid temperature range of the alloy (i.e., a temperature ranging between the liquidus temperature and the solidus temperature) during which time the material will have a varying solids fraction (fs), but will have a consistency more solid than liquid. The liquidus temperature is the minimum temperature at which all components of a mixture (such as a metal alloy) can be in a liquid state. Below the liquidus temperature, the mixture will be partly or entirely solid. The solidus temperature is the maximum temperature at which all components of a mixture (such as a metal alloy) can be in a solid state. Above the solidus temperature, some or all of the mixture will be in a liquid state.
This type of metal processing results in initiation nucleation in the mixture when the melt temperature of the mixture has dropped below the liquidus temperature, thereby preventing the normally-occurring formation of a dendritic (i.e., needle-like, crystalline structure in the material). During this type of metal processing and when the semi-solid material is at a specific solids fraction, the multi-constituent alloy material has a structure consisting of solid, spheroidal “α-particles” (i.e., solid particles having a higher melting point primary constituent) that are surrounded by a eutectic liquid (i.e., a liquid comprising lower melting point constituents). As such, during the processing of the mixture, the semi-solid material, hereinafter referred to as “SSM,” has a viscosity which enables the mixture to be handled as a semi-rigid mass. Such semi-rigid mass of the mixture can be injected into a mold under conditions of laminar flow, unlike the turbulent flow usually characterizing a conventional fully molten alloy forming and injection process.
Injecting SSM into a mold under laminar flow can eliminate many common defects associated with a conventional molten metal process for producing die castings, permanent mold castings and other casting methods. These defects include shrinkage porosity, formation of oxides, and gas porosity. Each of these defects can cause reduced mechanical properties of the formed mixture, such as lowered strength, reduced fatigue life, and/or reduced ability of the castings to satisfactorily be heat treated, which is typically employed to optimize strength and elongation of the casted product.
Another beneficial feature of a process that includes the shearing of a molten alloy is that after processing the SSM, the material can be allowed to fully solidify, and upon subsequent re-heating, the material retains the spheroidal “α-particle” SSM structure throughout its semi-solid temperature range. This latter reheating process has been a common and preferred practice due to the ability to create metal stock having the SSM structure through a high-volume bar casting operation. The bars can be readily shipped to a production facility, cut to a selected size, and then re-heated to a semi-solid condition in preparation for a forming or casting operation. However, this process is expensive due to costs associated with equipment used for reheating the SSM and casting the processed bar stock, as well as the inability to recycle processed material and scrap on-site while retaining the SSM structure.
Melting, cooling, and processing of SSM material on-site from standard raw metal stock can result in economies in both equipment and material recycling compared to the reheating process, in large part because expensive re-heating equipment is not required, and large quantities of material are not maintained in process—material that can be rendered unusable if an interruption of the heating or forming process should occur. This standard metal production process is performed with standard furnaces and molten metal transfer equipment. Scrapped metal can be readily recycled and reprocessed into an SSM condition on-site, as needed. Some cost is incurred for an on-site processing unit, but this is typically significantly less than the total cost of all the specialized equipment needed for the reheating process.
In both the reheating process and a conventional production process, the goal is to create a selected microstructure in the finished metal. A significant benefit of the SSM process and subsequent forming of the material into finished products is the ability for the viscous SSM material to flow in a laminar fashion into a mold, which minimizes the occurrence of defects.
Benefits from this process include improved mechanical properties and fatigue life, based on the minimization of oxides, gas porosity and shrinkage porosity. Safety-critical and pressure-sensitive components are prime candidates for these SSM forming processes.
Conventionally, the desired SSM condition is determined by the temperature of the molten metal charge using a thermocouple. The thermocouple is either immersed in the material, or embedded in the container holding the material. An alternate method is to retrieve a sample of material, and cut or knead the material with a spatula to get a “feel” for the viscosity. However, such alternate methods are imprecise, destructive, and involve a separate process step that is not “in line” with the essential melting and casting process. The thermocouple has limitations because it is effectively sacrificial, and can degrade, erode, or become fouled during use.
Other methods of process control are performed on a time basis, programmed via an algorithm that takes into account only the initial molten metal temperature through thermocouple sensing, and the known thermal characteristics of the metal alloy. This control method is unreliable since it does not take into account all variables, such as container temperatures and ambient temperatures. Existing methods also do not enable continuous monitoring of the condition of the SSM material throughout the charging, processing, delivery or transfer steps of the forming process.
A semi-liquid material, hereinafter referred to as “SLM,” also has a temperature ranging between the liquidus temperature and the solidus temperature, but with a consistency more liquid than solid. SLM is also utilized for forming and casting operations. Existing SLM methods also suffer from limitations. Such methods do not account for irregular forming cycle times caused by downstream machine interruptions, operator interruptions, or short-term maintenance interruptions. If a standard cooling cycle is delayed or interrupted, the SLM charge must be scrapped, and another metal charge must be processed when the machine interruption has been resolved. This can result in wasted material, and with attendant increased costs. Also, existing SLM methods are unable to control the SLM process in a manner that ensures that the temperature and viscosity conditions of the SLM charge are consistently the same for each metal charge.
In view of the current state of the art, there is a need for an apparatus and method that overcomes the past deficiencies associated with processing SSM/SLM. In particular, there is a need for an apparatus and method that can be used to control the SSM/SLM process in a manner that ensures that the temperature and/or viscosity conditions of the SSM/SLM charge are consistently the same for each metal charge, and which apparatus and method allows for continuous monitoring of the condition of the SSM/SLM material throughout the charging, processing, and delivery or transfer steps of a forming process.